Currently, induction loops located in the roadway are usually used to detect vehicles in places such as at a car park barrier. The roadway must be torn up to lay the induction loop and supply leads, and then repaved after these have been installed. Apart from the extremely cost-intensive installation required, induction loops are susceptible to damage, for example from shocks or if liquid penetrates the roadway through cracks. Furthermore, their operation is impeded by environmental conditions such as rain and fluctuations in temperature. Local factors, such as reinforced concrete in the roadway, also exert a negative influence on reliable operation.
It has therefore been proposed (EP 1193662 A1, DE 102 16 760 A1, German Patent Application 103 21 201.0) to use geomagnetic field sensors instead of induction loops to detect vehicles.
Because of the spherical detection characteristics of geomagnetic field sensors, however, they also detect vehicles in nearby lanes, for example vehicles in the lane behind the parking column at the barrier to a car park. There is no straightforward way to alter the directional detection characteristics of a geomagnetic field sensor so that it will only detect vehicles in the lane in front of the parking column. This is also a problem if the geomagnetic field sensor is located closer to the lane ahead of the parking column, or at least if a large, heavy vehicle is located in an adjacent lane on the other side of the parking column.
According to DE 101 23 223 A, a magnetic field sensor that detects changes in the combined magnetic field resulting from the geomagnetic field and superimposed static and dynamic background fields can be used to detect vehicles. The magnetic field sensor can be installed in a cavity in the roadway. It is provided with a permanent magnetic field in order to be able to establish a particular operating point.